1. Field of the Invention
The present invention relates to an ultrasonic flow meter for measuring a flow rate of a fluid flowing through a conduit with ultrasonic waves.
2. Related Art Statements
In known ultrasonic flow meters for measuring a flow rate of a fluid flowing through a conduit, an ultrasonic pulse is transmitted from an upstream position toward a downstream position within the conduit to detect a forward transit time of the ultrasonic pulse and an ultrasonic pulse is transmitted from a downstream position toward an upstream position within the conduit to detect a backward transit time of the ultrasoninc pulse, then a difference between the forward transit time and the backward transit time is detected to derive a transit time difference, and finally a flow rate of the fluid is measured from the thus detected transit time difference of ultrasonic pulse.
In a Japanese Patent Application Publication Kokai No. 2002-162269, there is disclosed a known ultrasonic flow meter, in which said transit time difference of ultrasonic pulse is detected using zero cross points. In a Japanese Patent Application publication Kokai No. 2002-243514, there is disclosed another known ultrasonic flow meter, in which said transit time difference of ultrasonic pulse is detected using cross-correlation of ultrasonic wave signals.
In the known ultrasonic flow meters, even when a single ultrasonic pulse is transmitted, ultrasonic receiving unit receives an ultrasonic wave over plural cycles as illustrated in FIG. 12 due to a self resonance of an ultrasonic vibrating unit. Therefore, the received ultrasonic wave contains a plurality of zero cross points. In the known ultrasonic flow meter using the zero cross point method, it is necessary to find corresponding zero cross points in the received ultrasonic wave of forwardly transmitted ultrasonic pulse and in the received ultrasonic wave of backwardly transmitted ultrasonic pulse.
However, if a signal-to-noise ratio of the received ultrasonic waves is decreased due to a descendant of signal level or if there is a rather large difference in amplitude between the received ultrasonic waves, the corresponding zero cross points might not be detected correctly. If the corresponding zero cross points could not be detected correctly, there might be introduced a large error in the measurement of flow rate.
In the known ultrasonic flow meter using the cross-correlation method, a received ultrasonic wave signal is treated statistically to derive a point at which a maximum cross-correlation is obtained as a measure denoting a transit time difference. A cross-correlation curve does not show a sharp curvature, and therefore in order to detect a maximum point accurately, it is necessary to approximate the cross-correlation curve by a polynomial expression such as a quadratic equation. FIG. 13 shows an example of such a calculation result using a cross-correlation and FIG. 14 shows a graph illustrating a peak position on an enlarged scale.
In the known ultrasonic flow meter using the cross-correlation method, a received ultrasonic wave signal of the forwardly transmitted ultrasonic pulse is sampled to derive a digital data series x composed of N sample values and a received ultrasonic wave signal of the backwardly transmitted ultrasonic pulse is also sampled to derive a digital data series y composed of N sample values. A cross-correlation between these two digital data series x and y is derived by the following equation (1).Rxy[m]=Σx[n]·y[n+m] (m=0, 1, 2, - - - , N−1)  (1)In this equation (1), Rxy denotes a cross-correlation, x[n], y[n+m] represent the data series x and y and Σ means an integration with n=1, 2, - - - N.
An amount of shift m at which the cross-correlation Rxy becomes maximum denotes a transit time difference between the forwardly and backwardly propagating ultrasonic waves. In order to detect such a shift amount m, a large amount of multiplications has to be calculated in accordance with the equation (1). When this calculation is carried out by software, it needs a long time period and when the calculation is conducted by hardware, it is necessary to use an expensive signal processing unit having a high performance.
A shift amount m at which the calculated cross-correlation Rxy becomes maximum is an integer, but a true maximum value of the cross-correlation is usually obtained at a middle point between m and m−1 or m+1. Therefore, in order to detect a true maximum value, the cross-correlation curve has to be approximated by a polynomial expression such as a quadratic equation as explained above.